In optical communication systems, optical switches are used to control the flow of optical signals One such optical switch is described in U.S. Pat. No. 4,152,713 to John A. Copeland, III et al, issued May 1, 1979, entitled UNIDIRECTIONAL OPTICAL DEVICE AND REGENERATOR. This device includes a body of a semiconductor material having four layers of alternating opposite conductivity type, i.e PNPN or NPNP, forming PN junctions between adjacent layers. Preferably, the inner two layers are made of a direct-gap semiconductor material, and the outer two layers are made of a wider-band-gap material so as to form a heterojunction between the outer two layers and each of the inner two layers. These heterojunctions act to confine electrons in the inner two layers where they are converted to photons. The photons are emitted from the device as a beam of light. The device is operated by applying a voltage across the device which is lower than the threshold voltage necessary in the dark to turn on the device and generate light. A beam of light is directed into the device which lowers the threshold voltage and thereby causes light to be generated in the device. The light is then emitted from the device as a beam. Although the device is turned on by directing a beam of light therein, it can only be turned off by removing the electrical input. Thus, to operate the device for optical communication purposes, the electrical input must be pulsed.
Another type of optical switch which can be turned both on and off by an optical signal is described in the application for U.S. Pat. Ser. No. 07/428,388, filed Oct. 27, 1989 by Jacques I. Pankove, entitled OPTOELECTRONIC SEMICONDUCTOR DEVICE. The device of this application includes a switch of a body of semiconductor material having four layers of alternating conductivity type, i.e. PNPN or NPNP, forming PN junctions between adjacent layers. Preferably, the inner two layers are made of direct-gap semiconductor material and the outer two layers are made of a wider-band-gap material so as to form heterojunctions between the outer two layers and each of the inner two layers. A first optically variable resistance device, such as a photoconductor, is connected in series with the switch and one side of a source of current, and a second optically variable resistance device, such as a photodetector, is connected in series with the switch and the other side of the current source. The switch is capable of emitting light when a voltage above a threshold is applied thereto. By directing a light into the first optically variable resistance device, the voltage applied across the switch is increased to a level just below the threshold and is raised to at least the threshold by directing a light into the switch so as to turn on the switch. The switch is turned off by directing a beam of light into the second optically variable resistance device which reduces the voltage applied across the switch below a holding voltage.
Thus, there are optical switches which can be turned on optically and off electrically and switches which can be turned both on and off optically. However, there are optical communication systems which require a switch which can be turned on by an optical input but which can be prevented from being turned on by a second optical input. Although the switch of the above application Ser. No. 07/428,388 has a provision for turning off, and therefore inhibiting the turn-on of the device, in the present invention the inherent amplification property of part of the device is utilized to greatly increase its sensitivity to the inhibit signal.